One configuration of a resilient torque-restraint mounting system is described in the commonly assigned U.S. Pat. No. 2,705,118 issued to M. G. Beck on Mar. 29, 1955. This configuration is shown as the "prior art" embodiment of FIG. 1. A pair of resilient hydraulic mounting units are located at substantially opposite sides of the torque axis of a first body. The hydraulic mountings, consist of an outer housing and an inner member and a movable elastomeric wall portion, moveable in response to vibrations thereto, and a fluid chamber within each hydraulic mounting. The working fluid is contained within the working chambers of the hydraulic mountings and within a conduit for connecting the two hydraulic mounting together. This system creates a resilient connection between a first body for retraining movements, and in particular for retraining torque relative to the second body. Depending on the orientation of the hydraulic mountings, this arrangement can provide a system which has both a low degree of freedom, i.e., stiff in the torsional direction and a high degree of freedom, i.e., soft in relative translation between the two bodies. Thus a system is provided which can react torque loads and yet remain soft enough in the vertical and lateral translation directions to provide adequate isolation.
The aforementioned fluid within the conduit can provide the means for a variety of tuning policies as is well known to those skilled in the art. U.S. Pat. No. 4,236,607 issued to Dennis R. Hawles and William A. Simmons on Dec. 2, 1980, describes the use of a liquid tuning mass, specifically liquid mercury, which provides the opportunity to generate amplified inertial forces to cancel vibration. Also taught is changing of the mass in the tuning passage by either changing the length, or cross sectional area of the fluid passage. The commonly assigned U.S. Pat. No. 4,811,919 issued to P. J. Jones on Mar. 14, 1989, is herein incorporated by reference and further teaches this tuning option. These tuning parameter changes will ultimately alter the fluid inertia value, or effective fluid mass contained in the fluid system. Thus, the system can be tuned to provide inproved isolation for any specified operating condition. In addition, as taught in Beck '118 patent, adding a restriction in the conduit can increase the damping level for controlling transitory conditions or for controlling torsional dynamic vibrations. This is accomplished by throttling the working fluid through the aforementioned restriction, as is well known to those skilled in the art.
The engine mounting system 9 shown in FIG. 2 is considered to be the current "prior art" torsionally stiff configuration, and includes a mechanical torque-restraint system 10. The FIG. 2 system consists of a torque tube 11 connected to the engine 13 and a structure 15 to which the various resilient mountings 17 and other attachments are connected. Torque from the engine 13 is reacted by torsionally winding up of the torque tube 11. The loads due to torque are transferred into the structure 15 through offset links 19 which are attached to levers 16 at each end of the torque tube 11. Since the links 19 are attached at a point spaced from the centerline of the torque tube 11, a moment is created for reacting the engine torque. Also, since the components in the mechanical torque-restraint system 10 are flexible, some torsional rotation of the engine 13 relative to the structure 15 will occur when torque is applied. This torsional windup results in deflections across the other resilient mountings 17 in the "prior art system".
Of further note is that most aircraft mounting systems are safetied or snubbed. These snubbers allow for a hard, metal to metal contact or stopping action for reacting conditions such as, large applied torques and large vertical or lateral loads. It is further notable that the stops are also used for supporting the engine once the resilient mountings 17 have reached their useful service life, i.e., they have drifted to the stops. All resilient mounts, especially elastomeric mounts, are subject to drift phenomena. Drift is a result of having a significant load, such as aircraft engine 13 weight, applied for long periods of time. Typically, the useful lifetime of the aircraft mount is determined by the time it takes to drift to the stops. Once the mounting has drifted to the stops, the mounting system no longer isolates effectively, i.e., the engine vibration is hardlined into the structure, and the mount needs to be replaced. The situation is further aggravated when a higher torque is applied simultaneously along with other vertical loading, such as upon take-off. The torque applied to the system will cause a torsional rotation of the engine, resulting in a translational deflection imparted to each resilient mounting 17. This deflection due to torque is superimposed on the mountings in the system along with those deflections resulting from normal loading, weight, etc. If the mounting condition is poor, this imparted rotational deflection may result in snubbing or metal to metal contact of at least one of the resilient mountings 17, whereas, the mounting may not be snubbed under just weight loading. Again as earlier noted, this snubbing causes excessive vibrations to be transmitted to the structure. Thus it can be seen, that any torsional rotation that occurs can effectively result in the need for prematurely changing out or replacing the resilient mountings 17. If the torsional deflection can be effectively minimized, a longer service life for the mounting can be obtained.
The "prior art" mechanical torque-restraint and "prior art" fluid torque-restraint systems described above offer significant benefits by having a relatively high torsional stiffness and yet still remaining soft enough in the translational directions, to provide adequate isolation. Also, as is known to those skilled in the art, the fluid torque-restraint system can be tuned to provide a variety of tuning policies such as tuning the fluid inertia to provide improved isolation, or tuning an orifice to provide increased damping. Yet, these prior art systems, when subjected to large applied torques, will still permit undesirable torsional motions of the engine to occur. Thus, it should be understood that, the current known mounting systems will, under large torques, be unable to restrain the undersirable relative torsional movements between the two bodies to which the torque-restraint system is attached. This will result in premature bottoming out or snubbing of the mounting system resulting in an increase in vibration transmitted under some conditions.